Duchenne and Becker muscular dystrophies (DMD/BMD) are caused by mutations in the gene encoding dystrophin,alargeproteincriticalforthemaintenanceofmyofibersandcardiomyocytes.Adirectapproach totreatingDMDwouldbetoreplaceorrepairthegenetoenableexpressionoftherapeuticlevelsofdystrophin and functional assembly of the dystrophin-glycoprotein complex. Gene replacement therapies based on systemic delivery of adeno-associated viral (AAV) vectors to delivery micro-dystrophin genes are showing encouraging promise for therapy and are being tested by several groups in pre-clinical and clinical trials. However, microdystrophins are not fully functional, and episomal AAV vectors are likely to be slowly lost duringnormalmuscleactivityandaging.Thislimitationcouldbeovercomebymethodstobypassorrepair dystrophinmutationsviageneediting.TheCRISPR/Cas9systemhaspotentialtoprovidehighlyspecificgene modification,andhasrecentlybeenshowntoinducelowlevelsofdystrophininstriatedmusclesofdystrophic mdxmice.However,one-thirdofallDMDcasesarisebyaspontaneousnewmutation,suchthatgeneediting strategieswillneedtobeadaptedforawidespectrumofgeneticlesionsthroughoutthe2.2megabaseDMD gene.WehaveexploredtheuseofAAVvectorstodeliveryCRISPR/Cas9cassettestomusclesofdystrophic mdx4cvmiceandshowthatmultiplestrategiescanleadtoexpressionofnearlyfull-lengthdystrophinatlevels upto25%ofwildtype.ThesepreliminaryresultsindicatethattheCRISPR/Cas9systemcanbeadaptedfor invivouse,buttheapproachneedsconsiderableoptimizationbeforeitcouldbeconsideredtherapeutically relevant.Weproposetotestmultipleparametersofthedeliverysystemtoenhanceefficiencywiththegoal of obtaining physiologically significant dystrophin & DGC expression without adverse events. These approachesincludeoptimizingdelivery&expressionofcomponentsoftheCRISPR/Cas9system,exploring timedandlimiteddurationdeliveryofCas9,andtargetingbothdifferentiatedmusclecellsandtheirstemcell progenitors.Ourinitialfocuswillbeondevelopingmethodstoachievedystrophingeneeditingatanefficiency needed to halt or reverse the pathophysiology in muscles of dystrophic mdx4cv mice. We will also adapt methodsdevelopedinmiceforAAV-mediateddeliveryofCRISPR/Cas9cassettestotheCXMDdogmodel ofDMD.Theabilitytoinducetherapeuticlevelsofdystrophininanimalmodelswouldenablefurtherstudies to limit genotoxicity and develop approaches for clinical translation. If successful, the studies could significantly advance the potential for clinical use of gene editing for DMD and other genetic disorders of muscle.